Motion and Energy

NET Force: The force that results when there is a number of forces of various amounts acting in various directions. ( For Example: The NET Force between you and your seat is zero. Your weight (force) acts toward the center of the earth but your seat is pushing up with a force called the Normal Force that is equal to your weight. The NET Force is zero, therefore your acceleration is zero, you are an object at rest which will stay at rest until the NET Force is not zero and you will move.

Newton's Laws

• 1st Law: Law of Inertia. Nothing changes unless a NET Force is in action.
• 2nd Law: The amount acceleration depends directly on the amount of Force applied to the object and inversely to the mass. The greater the Force the greater the acceleration. The greater the mass the less the acceleration. The acceleration is in the direction of the NET Force.
• 3rd Law: Action/Reaction. Push on an object and it pushes on you.

Class demos: String and Mass. Cut the String and the Mass falls. ( Change in the NET Force)
Two Students and Rolling Chairs. Hoop and Pen: Inertia Fan Cart Races with Masses.

Conservation of Energy: Energy cannot be created or destroyed - it just changes form.
Mechanical Energy:
Potential Energy = mgh
Kinetic Energy = 1/2mv²

For a pendulum:


Conservation of Energy: Energy at the release point = energy at the lowest point: mgh = 1/2mv², mass cancels so gh = v²/2.


Class Demo: Bowling Ball Pendulum. Drop a mass one meter. What is its velocity?
Estimation: v = squareroot(2 x 10 m/s² x 1m) = (20 m²/s²)^1/2 = 4.5 m/s.
From experiment: v = 10cm/.02 s = .01m/.02s
v = (1 * 10^-1)/(2 * 10^-2) = .5 * 10¹ = 5 m/s.

Energy: or Work is measured in Joules (MKS)
Power = Energy/time
The unit of power in the MKS system is the Watt, where a Watt = 1 Joule/1 second.

The power company charges you for kilowatt-hours (Kw-hr). Is the kilowatt-hour a unit of power or energy?
Power = Energy/time. A kilowatt-hour = (Power)(Time) = (Energy/Time)(Time) = Energy, so the kilowatt-hour is an energy unit.

How much energy is in a kw-hr?
1000 W x 1 hour = 1000 J/s x 1 hr = 1000 J/s x 3600 s = 1 * 10³ Joules x 3.6 * 10³ = 3.6 * 10⁶ Joules = 3.6 Megajoules = 3.6MJ.

Scientific Notation (See Appendix A in text)

New Unit: Important to know and understand when considering national energy sources:

That's a 1 with 15 zeros after it - one quadrillion.